V8 already depends on cppgc_base which means that unittests does not
need to depend on cppgc_for_testing any longer.

Move the cppgc_for_testing dependency to the stand-alone
cppgc_unittests binary

Bug: v8:10674
Change-Id: I07bfe30901eb1683d2e0ee0189f73b5244884f27
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2310249
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68970}